1. Field of the Invention
The present invention relates to a method and an apparatus for cutting a sheet member having a wire embedded therein, such as a steel breaker ply, a steel chafer, a steel carcass and the like for use in the production of a tire.
2. Description of the Related Art
For example, Japanese Utility Model Publication No. 1987-13779 discloses a cutting apparatus Which comprises a disk-shaped cutter provided above a conveyance path for a rubber sheet, and a cutter support bar disposed so as to be vertically movable and shiftable in a bias direction. The rubber sheet is cut from one side edge to the other side edge thereof by shifting the cutter in a direction orthogonal to the direction of the movement of the cutter support bar. But, the apparatus doesn't detect an error in the cutting angle and issue a warning.
For producing a tire with good precision, it is required to feed a tire forming machine with a tire rubber sheet cut very accurately.
Particularly, since a rubber sheet such as a breaker ply, a chafer, a carcass ply and the like is provided with twisted piano wires therein, the final tire product is adversely affected when the size of the sheet is inaccurate thereby causing dynamic disproportion in the tire.
Because the wires are cut at the opposite side edges in a width-wise direction of such a wire-embedded rubber sheet, the opposite side edges of the rubber sheet are covered to the extent of 10.about.20 mm with a raw rubber tape of 0.5.about.1.0 mm in thickness, referred to as edge rubber, so that the side edges are thicker than the other portions of the sheet.
Generally, a carcass ply and the like are provided at their middle portions, namely in the range of 50.about.70% of their width, with another rubber sheet of 0.5.about.1.0 mm in thickness adhered thereto.
Accordingly, when the cutter of the above-mentioned conventional cutting apparatus is placed on the wires at the side edge of the rubber sheet to be cut first, the ends of the wires tend to be untwisted. After shifting a while, the cutter tends to be diverted to a rubber layer having no wires due to the resistance of the wires. Otherwise, the cutter tends to become incapable of shifting laterally due to a large viscosity resistance of the thick rubber portion. Thereupon, cutting faults are occasionally caused at the side edge of the rubber sheet.
When being shifted from the one side edge of the sheet to the middle portion thereof, the cutter tends to rise in the transition portion of the sheet from the thin portion to the thick portion due to the increasing thickness of the rubber layer, thereby shifting without completing the cutting.
Further, the conventional cutting apparatus is incapable of automatically detecting the above cutting faults.
On the other hand, automation is highly desirable in the tire forming line. More specifically, an automatic cutting of the above-mentioned three kinds of rubber sheets, an automatic mounting thereof around a forming drum, an automatic piling of tire constituent materials around the rubber sheet already mounted around the drum and an automatic joining of the forward end and the rear end of the rubber sheet mounted therearound are highly desirable.
For accomplishing these automatic operations, it is necessary to always feed the rubbers sheets stably and accurately. Nowadays, a compensation mechanism or a control method for compensating for a variation in the width of the sheet has been proposed for attaining good accuracy, but no countermeasures for correcting an error in the cutting angle have been taken. The above-mentioned wire-embedded rubber sheets are produced carefully so that the wires are kept at a constant angle during production. But sometimes the wire angles are disarranged by the winding of the rubber sheet carried out for the storage or the feeding thereof. For ensuring the quality of a tire, the forward end and the rear end of the wire-embedded rubber sheet mounted around the drum should not be joined in a state in which the respective wires in the forward end and in the rear end intersect each other at the joined portion. In order to avoid such an intersection, the parallelism between both the wires in the forward end and those in the rear end should be maintained within a suitable tolerance.
This condition will become more comprehensible by the following explanation.
FIG. 7 shows the forward end and the rear end of a breaker ply after completion of the cutting. Only two wires are illustrated in an abbreviated manner. In the drawing, a black section 1 designates a twisted wire, and a white section 2 designates a rubber layer. A white section 3 designates a part of the rubber layer separated by the cutting, having an area about half that of the white section 2.
FIG. 8 is an explanatory view illustrating the tolerance for an error of the angles defined respectively by the forward end and in the rear end of a rubber sheet (the forward end is indicated with a solid line and the rear end is indicated with a broken line).
The symbols employed throughout these figures will be explained hereinafter:
p: pitch between wires embedded in the rubber sheet, PA1 d: diameter of wire embedded therein, PA1 .theta..sub.1 : angle of wire nearest to the forward end of the sheet PA1 .theta..sub.2 : angle of wire nearest to the rear end thereof, PA1 .theta..sub.3 : tolerance in the angle of the rear end of the sheet, PA1 W: width of the rubber sheet. PA1 d=0.68 mm PA1 p=1.40 mm PA1 W=200 mm PA1 .theta..sub.1 =20.degree.,
The forward end portion of the rubber sheet is indicated with the symbol A, and the wire nearest to the cutting line is indicated with the symbol A-1. The rear end portion thereof is indicated with the symbol B, and the wire nearest to the cutting line is indicated with the symbol B-1. The center of the wire A-1 is indicated with the symbols of C and F at the opposite sides, and the center of the wire B-1 is indicated with the symbols of D and E.
In a best or ideal condition, a parallelogram is defined by lines connected at the symbols of C, D, E and F with .theta..sub.1 =.theta..sub.2 as well as CD=EF=p/ Sin .theta..sub.1.
Even though the positions represented by symbols C and D can be actually located as illustrated in FIG. 8, the center E of the wire B-1 is often shifted to the point E due to an error in the angle. That is, the wire B-1 is not always disposed along the line DE, but is often disposed nearly along the line DE'. When carrying out a pressing operation of the end portions with a splice roll after the completion of the joining of the end portions, the present inventor has found out from experiments that a correction for an intersection between both the wires A-1, B-1 becomes more difficult as the center line of the wire B-1 gets nearer to line DE'.
By exemplifying how large of a difference can arise between the angle .theta..sub.2 of the wire B-1 and the angle .theta..sub.1 of the wire A-1 the importance of the present invention will become more comprehensible.
Now, one example of a radial tire for a passenger car has the following dimensions:
DE=DE'=W/Sin .theta..sub.1 =200/Sin 20.degree.=584.76 (mm).
When taking p=1.4 mm as a maximum tolerance in the line EE', an allowable angle error .angle.E'DE=.theta.' becomes 0.034.degree. because 2.times.584.76.multidot.Sin(.theta.'/2)=p=1.4.
That is, an angle difference between both wires A-1, B-1 for preventing the intersection therebetween becomes about .+-.0.03.degree.. Certainly, a smaller value may be derived according to the values of those W, .theta. and p.
Consequently, it is quite possible that a variation to such a degree may be caused before the rubber sheet is fed to the forming drum even when the rubber sheet is produced under strict production management.